Musannaf Ibn Abi Shaybah

Hazrat Zuhri says that for forty to one hundred and twenty goats, one goat is due as Zakat; then, when they exceed one hundred and twenty, two goats are due up to two hundred; then, if they exceed two hundred, up to three hundred, three goats are due; and if the goats exceed this, then for every hundred, one goat is due as Zakat, and less than forty is exempt.